The invention pertains to silicon nitride layers and to methods of forming silicon nitride layers. Additionally, the invention pertains to photolithography and etching.
Silicon nitride has many applications in modern semiconductor devices, as well as in modern semiconductor device fabrication processes. For instance, silicon nitride is electrically insulating, and is frequently utilized as a dielectric layer to separate electrically conductive circuit components. Additionally, silicon nitride is selectively etchable to other materials frequently used in semiconductor fabrication processes, such as, for example, polysilicon and silicon dioxide, and therefore makes an effective etch stop during etching of such other materials. It would be desirable to develop methods for altering chemical properties of silicon nitride to vary its insulative properties and etch properties for suitability to particular applications.
In addition to the above-discussed applications in which silicon nitride is commonly utilized, there are a number of semiconductor processing applications for which silicon nitride is not considered appropriate, or has less than desired attributes, for example, Si3N4 often requires discrete antireflective coating layers intermediate it and an overlying photoresist layer. Accordingly, Si3N4 does not have very good inherent antireflective properties. Antireflective coatings are utilized during photolithographic processing of photoresist layers to absorb light passing through the photoresist layers. Antireflective coatings can thereby prevent light from being reflected from beneath the photoresist layer to constructively and/or destructively interfere with other light propagating through the photoresist layer.
Although Si3N4 generally requires discrete antireflective coating layers intermediate it and an overlying photoresist layer, silicon enriched silicon nitride layers (i.e., silicon nitride layers having a greater concentration of silicon than Si3N4, such as, for example, Si4N4) frequently do not. However, silicon enriched silicon nitride is difficult to pattern due to a resistance of the material to etching. Silicon enriched silicon nitride layers are formed to have a substantially homogenous composition throughout their thicknesses, although occasionally a small portion of a layer (1% or less of a thickness of the layer) is less enriched with silicon than the remainder of the layer due to inherent deposition problems.
It would be desired to develop alternative methods of utilizing silicon nitride in wafer fabrication processes. As silicon nitride is relatively ubiquitous in semiconductor processing methods, it would be convenient if methods could be developed wherein the silicon nitride could function as an antireflective coating, and yet could also be relatively easily subsequently patterned.
In one aspect, the invention encompasses a method of forming a silicon nitride layer. A first portion of a silicon nitride layer is formed, and a second portion of the silicon nitride layer is formed. One of the first and second portions has a greater stoichiometric amount of silicon than the other of the first and second portions. The one portion comprises less than or equal to about 95% of a thickness of the layer of silicon nitride.
In another aspect, the invention encompasses a semiconductor fabrication process. A substrate is provided. A layer of silicon nitride is formed over the substrate. The silicon nitride layer has a thickness. A refractive index of a first portion of the thickness of the silicon nitride layer is increased relative to a refractive index of a second portion of the silicon nitride layer. The first portion comprises less than or equal to about 95% of the thickness of the silicon nitride layer.
In yet another aspect, the invention encompasses a semiconductor wafer assembly. The assembly includes a semiconductor wafer substrate and a layer of silicon nitride over the substrate. The silicon nitride layer comprises a thickness and two portions elevationally displaced relative to one another. A first of the two portions has less resistance than a second of the two portions, and comprises less than or equal to about 95% of the thickness of the silicon nitride layer.